Lasers have been used to perform cutting, etching, and other types of machining operations on various types of structures made of various types of materials. In particular, lasers can be used to micromachine complex patterns on structures. When a laser pulse hits a structure, the laser energy may cause material to be removed. A structure may be machined by using the laser to selectively remove material according to the desired pattern. The operating characteristics and parameters of the laser (e.g., wavelength, pulse rate, energy density) may be controlled to control the laser energy and thus the amount of material removed. Some structures, such as biomedical devices, may be difficult to machine using a laser because of the material and/or the shape of the structure.
In general, laser ablation is the removal of thin layers (usually sub micron) of material at low fluence levels, where the ablated material carries at least some of the residual heat away from the remaining workpiece. Fluence may be defined as energy density per laser pulse and energy density is generally defined as the intensity of an individual laser pulse focused onto a workpiece. The fluence multiplied by pulses per second is generally referred to as power density.
The ablation threshold is the practical energy density where detectable amounts of material begin to be removed. When the energy density is above the ablation threshold of a material, the material will be ejected and will carry away excess or residual heat energy. Below the ablation threshold, the laser energy is converted into heat within the material. As the ablation depth increases, the energy density threshold may increase and the residual heat penetration may increase (i.e. the laser energy transmitted into the depth of the material not ablated). In general, optimum fluence is the fluence level that provides the highest efficiency of material removal and the lowest percentage of residual heat left in the material after ablation. Excessive fluence is the higher range of fluences where the etch rate becomes saturated and excess energy is converted into heat in the remaining material adjacent to and below the etched or ablated volume.
Residual heat penetration during ablation may affect the resolution of the features that are machined. In general, high resolution is ability to generate fine features, for example, using laser ablation with little or no detectable melting as compared to the sizes of the features desired. Heat free or cool excimer laser ablation may use low energy densities to strip fine (e.g., sub-micron) layers with high resolution such that excess heat may be carried away primarily in the ablated materials and the residual heat left in the remaining materials is low enough to substantially eliminate melting.
Certain types of materials may be more difficult to machine with lasers than others. Sensitive materials are normally not easily processed at longer wavelengths (e.g., at 248 nm) due to optical and thermal effects. Sensitive materials generally have an optical absorption at the 248 nm wavelength that is low compared to the optical absorption at the 222 nm, 193 nm or 157 nm wavelengths. Sensitive material may also have a relatively low thermal capacity and/or low melting point, i.e., it will melt and deform with low heat input (or low power density). Examples of sensitive materials include, but are not limited to, polymers, other low density organic materials such as PMMA, collagen, living tissue such as the cornea, resorbable polymers, types of nylon, delrin, PET, Mylar and the like.
As an example, a sensitive material, such as PMMA, has an OAC of about 65 cm−1 at 248 nm and about 2.0×103 cm−1 at 193 nm, and also has a relatively low melting temperature. Using existing laser machining systems, PMMA may be effectively processed at 193 nm with good resolution at moderate power densities but may only be marginally processed at 248 nm. In contrast, a non-sensitive material that may be processed at 248 nm, such as polyimide, has an optical absorption coefficient (OAC) of about 2.8×105 cm−1 at 248 nm and about 4.2×105 cm−1 at 193 nm. Polyimide can be effectively processed, with little or no melting, at high repetition rates (i.e., >400 Hz) at 248 nm due to its strong absorption at that wavelength. Polyimide also has a relatively high melting temperature and can also withstand higher power densities.
When a laser is used to machine thermally sensitive polymers, the material surrounding the area being machined has a tendency to melt or deform under laser irradiation. During laser ablation, the low optical absorption coefficient (OAC) of the sensitive material results in a deeper penetration depth of the laser energy into the material and therefore a larger volume of material is removed with each laser pulse, which may require the laser fluence (i.e., energy density per laser pulse) to be higher. In other words, if the absorption depth of a material is relatively large (e.g., as compared to a heat free excimer laser processing condition), the energy density required to ablate the material also increases. The longer absorption depth coupled with the higher required ablation threshold energy density may result in a larger etch depth per laser pulse and may also result in a larger residual heat which consequently remains in the material adjacent to the etch zone after ablation.
To avoid damage or degradation due to thermal effects, therefore, some laser machining techniques have been carefully designed to use a specific wavelength and/or energy density that will minimize thermal effects. In some applications, for example, a wavelength of 193 nm is used to process sensitive materials, such as resorbable polymers, because of the stronger absorption at the shorter 193 nm wavelength. Machining using a 248 nm laser, however, has other advantages over a 193 nm laser. A 248 nm laser produces higher laser power for essentially the same cost as a 193 nm laser. The beam delivery system of a 248 nm laser may be less expensive due to the ability of 248 nm to be transmitted through air while a 193 nm laser may require a sealed and N2 purged beam delivery system. The beam delivery optics for a 248 nm laser generally cost less and last longer than for 193 nm lasers. The operation cost of a 248 nm laser may also be lower due to a longer life of the laser resonator and beam splitter optics, the laser tube component and the laser gas fill and due to lower stress on high voltage components. The 248 nm laser may also be intrinsically more stabile in terms of power and energy fluctuations than the 193 nm laser.
Thus, a 248 nm laser may provide advantages in laser machining applications but may have drawbacks when used to machine sensitive materials. In particular, the achievable resolution may be limited due to the melting effects. Sensitive materials may be effectively processed using a 248 nm laser at low power density, (i.e., low laser pulse repetition rates) if the power density is low enough to allow time for the residual laser energy to dissipate within the material. However, this limits the effective processing speed for the sensitive materials and therefore limits the economics for high volume production applications.
Increased power density directly relates to material heating and therefore the melting and thermal effects generated during laser processing. Power density can be reduced, for example, by reducing the pulse repetition rate or by reducing the amount of time the material is exposed to the high repetition rate laser. Thus, a fast scanning speed of the workpiece under a laser beam can reduce the effective, local power density delivered to that workpiece. When using fast scanning speeds, however, the ability to image complex non-repeating features over a large area may be difficult due to the optics required to image a large area with high optical resolution. Machining complex patterns may be even more difficult on certain shapes, such as a curved surface on a cylindrical structure.
In addition to the challenges presented by certain materials, laser machining of certain types of structures has also presented problems. Using laser micromachining to create three-dimensional (3-D) structures on a curved surface, for example, has been difficult to achieve with existing systems and methods. In particular, when the size of a 3-D structure is over a few millimetres, conventional near field imaging from an excimer laser may not properly create the large structure due to its limited maximum field of view.
Accordingly, there is a need for a system and method for laser machining 3-D structures on a workpiece made of a variety of materials.